Adult-onset Still's disease (AOSD) is a clinical entity with a heterogeneous etiology. We have encountered patients with AOSD who had severe polyarthritis and who fulfilled the classification criteria for rheumatoid arthritis (RA); however, most patients with AOSD typically exhibit mild arthritis. In this study, we proposed 2 clinical subsets of AOSD and investigated the clinically significant characteristics of the 2 subtypes.


We retrospectively analyzed 71 consecutive patients with AOSD. We reviewed the medical records of all patients who were followed up for more than 2 years. We classified all of the patients with AOSD into the following 2 subsets: an RA subtype for patients who met the criteria for RA according to the American College of Rheumatology and a non-RA subtype for patients who did not meet the criteria for RA.


Our results indicated that the non-RA subtype was accompanied by severe inflammatory complications, including pleuritis and hemophagocytic syndrome. In addition, the serum ferritin and serum interleukin-18 (IL-18) levels were significantly higher in patients with the non-RA subtype than in those with the RA subtype. Interestingly, only 1 patient with the RA subtype had anti–cyclic citrullinated peptide antibodies and 1 patient with the non-RA subtype had rheumatoid factor. These findings distinguish these patients from patients with true RA.


There were 2 subsets of patients with AOSD in the examined population. Patients with high levels of IL-18 or ferritin presented with severe systemic inflammatory disorders (non-RA subtype) and patients with low levels of IL-18 or ferritin developed severe arthritis (RA subtype).